The technique disclosed herein pertains to seismic surveying and, more particularly, to marine seismic surveying at low frequencies with deeply towed, heavy seismic sources.
The pursuit of hydrocarbons and some other fluids is sometimes greatly hampered by their being located in deposits underground in certain types of geological formations. Such deposits must be identified and located by indirect, rather than direct, observation. This includes imparting acoustic, or sound, waves of selected, seismic frequencies into a natural environment so that they may enter the earth and travel through the subterranean geological formations of interest. During their travels through the formations, certain features of the formations will return waves back to the surface where they are recorded. The seismic data thus recorded contains information regarding the subsurface geological formations from which one can ascertain things like the presence and location of hydrocarbon deposits. That is, seismic data are representative of the geological formations from which they are obtained.
For example, one tool frequently used in the analysis of the seismic data is what is known as a “velocity model”. A velocity model is a representation of the geological formation that can be used in analysis. It may be used to, among other things, convert the seismic data into one or more “seismic domains” that image the geological formation in different ways. The quality of these images frequently depends upon the quality of the velocity model. It may also be used in other ways to, for another example, analyze various geophysical characteristics of the formation. Other types of models of the underlying geological formations, collectively called “subsurface attribute models” herein, are also used and implicate similar considerations in the present context.
Over time, the need to locate hydrocarbon deposits more accurately and more precisely has grown. Sometimes advances in accuracy and precision come in the form of new acquisition techniques. Other times such advances are achieved through the manner in which the seismic data are processed such as those described in the above. Sometimes advances result from a combination of developments in both acquisition and processing.
A relatively recent development in seismic acquisition is “low-frequency” acquisition. Seismic surveying historically has used frequencies in the range of 8-80 Hz for seismic signals because of their suitability in light of technical challenges inherent in seismic surveying. The term “low frequencies” is understood within this historical context as frequencies below which getting sufficient signal to noise with conventional sources rapidly becomes more difficult as the frequency decreases, i.e. below about 6-8 Hz.
The use of low frequencies for imaging marine seismic data has proven challenging for frequencies below about 6 Hz, particularly for frequencies below about 4 Hz. The challenge is twofold: (1) at lower frequencies, the naturally occurring seismic background noise of the Earth gets progressively stronger and (2) conventional broadband sources such as airguns get progressively weaker. As a result, the signal-to-noise of deepwater marine seismic data can decline at over 30 dB per octave for frequencies below 4 Hz.
Thus, while there may be many suitable techniques for seismic imaging in general and for generating subsurface attribute models in particular, the need for increased effective signal-to-noise ratios, at low frequencies, continues to drive innovation in the art. In particular, among other things, there is a need for acquisition and processing techniques that enhance acquisition and use low-frequency seismic data at lower frequencies. The art is therefore receptive to improvements or at least alternative means, methods and configurations that might further the efforts at improvement. As a result, the art will welcome the technique described herein.